1. Field of the Invention
The present invention relates to a sensing element for a nitrogen oxide molecule, a sensing apparatus for a nitrogen oxide molecule, and a method for sensing a nitrogen oxide molecule.
2. Description of Related Art
Nitrogen oxide (NO) is oxidized in the atmosphere to turn to poisonous nitrogen dioxide. Thus, it is desired to sense the state where NO at a low concentration is present. It is known that NO is a neurotransmitter in living bodies, and fulfills an important function in immunoreaction, blood pressure adjustment, and the like. As described herein, attention has been paid to the sensing of NO for the environment, or the diagnosis of living bodies.
As a conventional NO molecule sensing apparatus, known is an apparatus in which a monolayer of a derivative of a porphyrin molecule (hereinafter abbreviated as FeP) having, as a central metal, an iron atom is used as the gate region of an FET (field effect transistor) (see, for example, Non-Patent Literature 1: D. G. Wu, G. Ashkenasy, D. Shvarts, R. V. Ussyshinkin, R. Naaman, A. hanzar, and D. Cahen, “Angew. Chem. Int. Ed”, WILEY-VCH Publisher, May 17, 2000, pp. 4496-4499). A NO molecule is coordinated and bonded to the central metal of FeP, whereby the electronic state of FeP changes. This change in the electronic state causes a change in static electricity of the gate region so that the drain current (concerned) is changed. In such a way, an FET type sensor senses a NO molecule in accordance with a change in the drain current. Non-Patent Literature 1 discloses that the lowest concentration that can be sensed is about 4.1 ppm.
In the meantime, as an apparatus for directly measuring a change in the electronic state of a molecule, known is an apparatus of estimating the electric conduction characteristic of a single molecule junction between electrodes (see, for example, Patent Literature 1: JP-A-2005-127998). This is a molecule-sensing nanosensor having a sensing portion made of bipyridine. When a platinum ion is bonded to the sensing portion, bipyridine is changed into an electroconductive state. The molecule-sensing nanosensor is a sensor of detecting a change in the electronic state of its sensing portion.
Also known is an apparatus of estimating the electric conduction characteristic of a porphyrin molecule junction between electrodes (see, for example, Non-Patent Literature 2: K. Tagami and M. Tsukada “e-Journal of Surface Science and Nanotechnology”, Published by the Surface Science Society of Japan, Jun. 30, 2003, pp. 45-49). This reference estimates the electric conduction characteristic of a polymer called tape porphyrin, wherein porphyrin molecules are bonded to each other. In Non-Patent Literature 2, as a functional group bridging tape-porphyrin and electrodes, the following three species are each prepared and their electric conduction characteristic are compared with each other: a mercaptophenyl group, a mercaptophenylenevinyl group and a mercaptophenyleneethynyl group. In tape-porphyrin, even when changing its functional group to the above groups, the electric conduction characteristic is not very largely changed among the apparatuses using these species.